Here’s another case where an older vehicle needs good old-fashioned diagnosis to find the cause of a problem. Follow this Real Fix to see how a lean condition on this Mercury Sable was diagnosed and repaired.
Vehicle
1995 Mercury Sable LS 3.8L
Title
Code 172, Replaced Fuel Pump
Complaint
The customer states the check engine light is on.
Cause
Connected a scan tool and found code 172 – System Indicates Lean (Bank 1). Off idle, backprobed the oxygen sensor (B1-S1) and oxygen sensor (B2-S1) signal wires and monitored the sensor signals on a lab scope. Observed the oxygen sensor (B1-S1) and oxygen sensor (B2-S1) signal parameters were below specifications. Performed a propane enrichment test at idle, monitored the oxygen sensor (B1-S1) and oxygen sensor (B2-S1) signal parameters on the lab scope and found the sensor signal parameters responded to the rich fuel mixture condition and idle quality improved significantly. Connected a smoke machine to the air intake system, induced smoke into the system to check for leaks and found no leaks. Connected a fuel pressure gauge to the fuel filter inlet, tested fuel pressure with the engine at idle and found the measured fuel pressure was below the specified range of 35 to 45 PSI. Visually inspected the fuel tank module wiring harness and found no obvious signs of damage. Used a multi-meter to check for the presence of voltage and ground at the fuel pump and found both were present. Disconnected the high pressure fuel line and placed the end of the line in a suitable container for further testing. Used a set of fused jumper wires to command the fuel pump on for 30 seconds and monitored the volume of fuel exiting the high pressure fuel line. The measured volume of fuel in the suitable container was below the specified 1.1 pints after the specified test interval. These test results verified the fuel pump was faulty.
Correction
Replaced the fuel pump, cleared codes, performed a road test and verified the vehicle operated properly.